Tuesday, 26 May 2015

Megafruit pining for absent megafauna

From a distance this 25 metre high tree looks like a flowering gum of some kind, a bloodwood (Corymbia) perhaps. The large leaves and, in February, the clusters of pink flowers are quite misleading.

Sort of. It's the right family, Myrtaceae, but wrong genus. This is a Lilly Pilly, Syzygium moorei. The fruits, when they form in March, give it all away. As does the location of the flowers and the fruits, clustered around the stems and branches. In a flowering gum they would be at the ends of branches.

This type of flowering and fruiting is called cauliflory, which I've mentioned before in passing. Why plants do this probably depends on the particular species and its pollinators, but it happens most commonly in tropical or subtropical rainforests, the natural habitat of our species.

Flowers packed around the branches are usually more firmly attached and less likely to be removed by a large pollinating creature, such as a bat or mammal. In this mature tree all the flowers are well above the grazing height of a ground-dwelling animal, but well place for climbers and creepers.

Syzygium moorei is called by local Aboriginal people the Durobby or Coolamon, Europeans have named it Rose Apple (a name applied to a few lilly pillies) or Watermelon Tree. The last name gives you a hint about one of its distinctive features - big fruits. Although not that (watermelon-sized) big. These fruits still had a little growing to do when I photographed them in in early May, but they are close to their final size of 3-5 centimetres in diameter, and their final colour - white.

The relative bigness of the fruits got scientists thinking about what might have eaten then before humans settled in Australia over 60,000 years ago.

Lui Weber speculates on this in an article in Wildlife Australia about plants not doing so well after the demise of Australia's megafauna. He suggests that Durobby is a 'rainforest tree with big fruits and a tiny subtropical distribution (Richmond River to Mudgeeraba area), consistent with it being a plant that depended on now-extinct cassowaries for dispersal'.

So where are the cassowaries in Australia? They have a fragmented distribution north of Townsville up into Cape York. These days they certainly don't find there way down to the Gold Coast in southern Queensland, the north-east corner of New South Wales,

Weber is worried about the future of Durobby and other rainforest species in Australia. In the absence of a big animal to eat the fruits and disperse the seed (out their rear...) the advantage of producing relatively few, larger seeds becomes a liability. All that energy and risk without a suitable animal distributor. Relatives and neighbours of the species produce small seeds and are still well served by the local animals.

Where did the 'megafauna' go? In the same issue of Wildlife Australia, fellow Australian ecologist Chris Johnson concludes that while the drying climate may have hastened the decline of rainforest species, many big herbivores found it a positive change, giving them opportunities to evolve and spread into new habitats. Overall it was most likely hunting by humans that sent the megafauna to extinction. This over time, may send some of our megaflora the same way.

In Africa, a reduction in the number of elephants due to habitat destruction and hunting is having a similar impact on large-seeded rainforest plants, which are now finding it difficult to regenerate and disperse. In Australia, the White Bark (Endiandra compressa) is distributed widely throughout the remaining tropical rainforest up north where the cassowaries distribute its seed, while in the south it clings to the side of streams where it probably relies on 'water, rodents and gravity'.

There are fossils leg bones of a cassowary species in southern Queensland, a dwarf species like that found in New Guinea today. This may have been our plant's megafaunal companion. Weber suggests that 'one way to help [plants like the Durobby] would be to reintroduce cassowaries to southern Queensland and northern New South Wales'. The next decision would be whether it's the larger northern Queensland species or its smaller relative in New Guinea.

A dwarf or regular-sized cassowary couldn't reach the fruits on our Durobby so presumably they fall to the ground first. In the Royal Botanic Gardens we have no plans to introduce Cassowary, or for that matter elephants.

Just come back from the Daintree where we saw Cassowaries in the wild. They are spectacular and we were lucky enough to see the fruits that they swallow whole lying all around the ground - a veritable smorgasboard! Should we be trying to grow these sorts of plants in an area such as Melbourne if we don't have the right ecosystem to sustain them?